This invention concerns an improvement of fiberglass tubular threaded joints, more specifically, to a fiberglass tubular coupling with a thermoplastic liner.
Composite fiberglass tubulars, such as line pipe and downhole tubing, are attractive for use in an oil field because of their high degree of corrosion resistance. Typically, steel tubulars are used in the oil field market. These steel tubulars have threads at each end for the purpose of joining sections of the tubing. Assembly of the threaded joints is accomplished by applying thread dope to the threads and then threading the joints using substantially high torque. Steel threads deform elastically under high torque conditions and form a leak tight seal which can be disassembled and reassembled several times. The problem with steel tubulars, however, is that they are not corrosion resistant.
Composite fiberglass tubulars have not been able to penetrate the oil field market and corrosion-resistance service to the extent predicted due to the problems associated with joints. The problem is created because of the rigidity of the threads at the end of the fiberglass tubulars. Fiberglass threads do not deform appreciably and thus are more difficult to seal and are much more susceptible to damage under high torque conditions. High torque may cause the threads to crack and also tends to cause thread compounds to seize in the threads leaving "ground glass" surfaces wedged together which are extremely difficult to break apart. Therefore, composite fiberglass tubulars have proven difficult to disassemble, and reassembly is not assured due to damage done to the threads.
It is, therefore, desirable to provide composite fiberglass tubular couplings with elastically deformable threads for use in high torque conditions.